A phase shift mask, in which projected light that passes through adjacent patterns has a phase difference of 180 degrees, is employed in the lithography process of a semiconductor integrated circuit. In addition to the main pattern, a fine assist pattern called SRAF (Sub Resolution Assist Features) is used in the phase shift mask. The main pattern is transferred onto the wafer. The assist pattern utilizes the interference of the light to improve the transferability of the main pattern. The assist pattern is not transferred onto the wafer. The method for generating pattern data in a computer is implemented to obtain the phase shift mask including the assist pattern. It is desirable for such a method to generate high-precision pattern data.